The invention relates to a strainer arrangement for straining liquid from a mixing arrangement, such as a mixing tank, for black liquor containing solid particles of ash. The ash in the black liquor is formed during combustion of black liquor and is separated from the flue gases and is mixed with new black liquor for introduction into the combustion installation which usually consists of a recovery boiler. For re-introducing the ash into the recovery boiler, it is important that the ash is thoroughly mixed with the black liquor which is to be combusted and that larger particles of ash, which may obstruct the nozzles for injecting the black liquor into the recovery boiler, are strained off using a suitable arrangement, and the invention relates to such a strainer arrangement.
Mixing ash separated from the flue gases with black liquor which is to be combusted in a recovery boiler is a well-known technique which has been practised for many decades. Various installations for performing such mixing have been designed over the years. Such mixing arrangements are called mixing tanks and usually consist of a vessel with agitator(s), provided with liquor inlet(s), liquor outlet(s), ventilating pipe(s), ash inlet(s) and in most cases a heating arrangement. As agitators, use has as a rule been made of slow-moving agitators with arms which sweep along the perimeter of the mixing tank and create no more turbulence than allows larger particles to settle to the bottom zone. The lowest arms of the agitator are therefore made in the form of a scraping unit which works close to the bottom and performs mechanical treatment there of the settled material which is crushed into smaller particles which can gradually rise up to a higher level in the tank and pass out through the outlet.
In order that the liquor distribution in the combustion chamber takes place in a trouble-free manner, the injected liquor must not contain particles which are too large because these may block the injection nozzles. The liquor is therefore usually strained through a fixed strainer known as a perforated plate which is placed in front of the outlet of the mixing tank.
One example of a modern embodiment of a mixing tank is described in Swedish Patent 9600100-3. Good mixing is achieved in this mixing tank with the aid of baffles on the agitator and stationary baffles on the bottom of the tank. Via openings located vertically one above another and provided with a strainer plate on the outside, the mixed liquid can flow into a vertically positioned container of semi-circular section on the outside of the tank, from where the liquid is pumped onward.
The strainer plate, which constitutes a part of the outside of the tank and is lowered into vertical holders and which covers a large number of outlet openings, strains off larger particles and functions well as long as the strainer plate has not been obstructed. From time to time, however, it has to be cleaned and this can be done by lifting it up and subjecting it to cleaning outside the mixer or by liquid being allowed to flow in counter-current or by other means. However, this is a complicated procedure which disrupts production. For this reason, it has long been desirable to be able to produce a strainer arrangement which is cleaned automatically and without interrupting operation.
According to-the present invention, a strainer arrangement for straining liquid from a mixing arrangement, e.g. a mixing tank, for black liquor containing solid particles of ash has been produced, which strainer arrangement is characterized in that it comprises a strainer pipe with gaps for radial inflow into the strainer pipe of liquid, and also movable cleaning pins in the gaps.
According to the invention, it is expedient that the gaps extend axially, preferably vertically, and that the cleaning pins are mounted on an axially movable shaft in the strainer pipe.
According to the invention, the cleaning pins may be mounted on the shaft via rotatably suspended holders resembling wheels.
According to the invention, the cleaning pins may have a round or angular cross-section.
According to the invention, the gaps should be distributed uniformly around the perimeter of the pipe and expediently total twenty-four in number.
According to the invention, the gaps expediently have a length of roughly 500 mm and/or a width of roughly 8 mm and are preferably arranged in a number of groups, expediently four groups, one above another.
According to the invention, the movements of the shaft can be brought about with the aid of an electric motor or pneumatically or hydraulically.
According to the invention, in order to bring about the vertical movements, an extension shaft coupled to the uppermost part of the shaft is provided with trapezoidal threads to make worm-gear driving possible.
According to the invention, a seal is expediently arranged around the shaft at the upper end of the strainer pipe.
According to the invention, the strainer pipe may be arranged in a vertical space on the outside of the tank wall, freestanding from the mixing tank or inside the latter, or it may consist of a strainer plate on the tank wall.